1. Field of the Invention
The invention relates generally to exact image reconstruction in a cone beam computed tomography imaging system having a radiation source scan path that encircles a region of interest (ROI) in an object, and more specifically to such a cone beam imaging system wherein the source scan path does not require circle scans at the ends of the ROI.
2. Description of the Prior Art
Recently a system employing cone beam geometry has been developed for three-dimensional (3D) computed tomography (CT) imaging that includes a cone beam x-ray source and a 2D area detector. An object to be imaged is scanned, preferably over a 360.degree. angular range and along its entire length, by any one of various methods wherein the position of the area detector is fixed relative to the source, and relative rotational and translational movement between the source and object provides the scanning (irradiation of the object by radiation energy). The cone beam approach for 3D CT has the potential to achieve 3D imaging in both medical and industrial applications with improved speed, as well as improved dose utilization when compared with conventional 3D CT apparatus (i.e., a stack of slices approach obtained using parallel or fan beam x-rays).
As a result of the relative movement of the cone beam source to a plurality of source positions (i.e., "views") along the scan path, the detector acquires a corresponding plurality of sequential sets of cone beam projected data (referred to hereinafter as cone beam data), each set of cone beam data being representative of x-ray attenuation caused by the object at a respective one of the source positions.
There are two basic approaches for processing the acquired cone beam data for reconstructing a 3D image of the object. The traditional approach is "Radon space driven", wherein the acquired sets of cone beam data are processed so as to develop a multitude of individual samples of Radon data for filling a Radon space "region of support" for the object. Once a sufficient amount of Radon space data has been developed, contributions to the final image reconstruction can begin. A newer approach is "detector data driven", where the individual sets of the acquired cone beam data (i.e., the detector data) are processed for directly developing successive contributions to the final image reconstruction (i.e., without the requirement of being transformed through Radon space). No matter which approach is used, the "processing" of the acquired cone beam data begins with a step that calculates line integral derivatives for a plurality of line segments L formed in the cone beam data. Detector driven image reconstruction processing is generally more desirable than Radon space driven processing because image reconstruction is more directly developed and therefore less system memory is required. In either case, the acquisition of the cone beam data is complete only if it can be processed so as to reconstruct the object with the desired resolution, and without artifacts. In the Radon space driven approach, this means developing a sufficient density of Radon data in a so-called "region of support" (a region which topologically corresponds to the field of view occupied by the region of interest in the object in real space). Typically, sufficient Radon data is acquired by exposing the entire object within the field of view to the radiation source.
Sufficient filling of the Radon space by cone beam CT apparatus having various scanning trajectories (paths) and using an area detector having a height which is less than the height of a region of interest (ROI) in the object being imaged, are known for performing an exact image reconstruction. For example, U.S. Pat. No. 5,463,666 entitled HELICAL AND CIRCLE SCAN REGION OF INTEREST COMPUTERIZED TOMOGRAPHY issued Oct. 31, 1995, describes a Radon space driven technique for imaging an ROI in an object without blurring or artifact introduction by providing a scan path consisting of a central spiral portion having a circle portion at its upper and lower ends, respectively, which are level with upper and lower boundaries of the ROI in the object. The witch from a spiral scan path to a circular scan path is necessary in order to obtain complete cone beam data at the upper and lower boundaries of the region of interest without overlap, thereby avoiding image blurring or artifacts which result from imaging portions of the object that are not within the ROI, as described in greater detail in the forenoted U.S. Pat. No. 5,463,666.
Although the above and other techniques have been useful, they require scan paths having both spiral and circular path portions, and necessarily results in abrupt shifts in movement of the scanning equipment portion of the imaging apparatus. Such abrupt shifts in scan movement are undesirable in that they either subject the patient to undesired jostling, or subject the scanning equipment to extra mechanical stress. It would be desirable to provide only a smoothly changing scan path. Furthermore, due to the use of circle and spiral scan path portions, a rather complex signal processing technique is required in order to avoid the combination of overlapping data on a given integration plane. It would be desirable to reduce the complexity of such signal processing required for image reconstruction.
U.S. patent application Ser. No. 09/052,415 entitled PRACTICAL CONE BEAM IMAGE RECONSTRUCTION USING LOCAL REGIONS-OF-INTEREST, filed Mar. 31, 1998, describes a Radon space driven technique for imaging an ROI in an object wherein the imaging equipment does not have the requirement to provide circle scans at the top and bottom of the ROI. As described therein, this is accomplished by processing the acquired cone beam data to develop specific sub-sets of Radon space data. Each of the sub-sets of Radon space data is targeted for reconstructing a corresponding "local 2D ROI" in a 2D parallel projection of the object on one of a plurality of vertically oriented coaxial .phi.-planes that partitions the Radon space. After a targeted sub-set of Radon data is developed, it is subjected to a first inversion processing step for developing the corresponding local 2D ROI projection image. Thereafter, multiple ones of the local 2D ROI projection images are grouped together and subjected to a second inversion processing step, thereby developing an image reconstruction of a portion of the 3D region of interest in the object. This process is repeated until the entire ROI in the object is reconstructed. By targeting specific subsets of Radon data for processing, the local 2D ROI projection images do not suffer from data corruption from overlaying objects, thereby allowing the use of a simplified scan path having only a spiral portion for traversing the ROI. Although this image reconstruction technique simplifies the scan path, reduces the memory requirements and improves the speed of the imaging system, the image reconstruction processing is still rather complex and the system memory requirements are still great.
U.S. patent application Ser. No. 09/052,281 entitled EXACT REGION OF INTEREST CONE BEAM IMAGING USING 3D BACKPROJECTION, filed Mar. 31, 1998, incorporated by reference herein, describes a detector driven technique called Filtered Backprojection (FBP) for reconstructing a 3D image of an ROI in an object. In this 3D backprojection technique, a first image reconstruction processing step calculates line integral derivatives for a plurality of line segments L formed in the cone beam data acquired at each source position. The extent of the line segments L in the data acquired at each of the source positions is determined by a data combination mask having its upper and lower bounds formed by cone beam projection onto the plane of the detector of portions of the source scan path that are above and below the source position that acquired the cone beam data in which the line integral derivatives for the line segments L are being calculated. In the next step, the calculated line integral derivatives for the line segments L are backprojected onto a 2D space corresponding to the plane of the detector. In the last step, the results of the 2D backprojecting step are 3D backprojected into a 3D space, each 3D backprojection provides a partial reconstruction of a 3D image of the object. Although the imaging equipment is still required to provide a circle scan path at the top and bottom of the ROI in the object to avoid image blurring or artifact introduction, the described masking/3D backprojection detector driven technique greatly reduces the complexity and memory requirements for image reconstruction signal processing as compared with Radon space driven image reconstruction techniques.
It would be desirable to provide exact image reconstruction processing which does not have the requirement of circle scan paths at the top and bottom edges of the ROI, and it would be particularly desirable that such processing be detector data driven. Such image reconstruction processing would be exact, less complex than the current Radon space driven processing, and subject neither the imaging equipment nor the patient to the abrupt shifts and stress of a segmented scan path.